The invention relates to low-VOC/VOC-free compositions of dialkyl-functional alkoxysiloxane oligomers, especially of methylalkylalkoxysiloxanes, to processes for producing them and to their use for hydrophobizing porous mineral substrates, in the course of which few or no volatile organic compounds are released.
It has long been known that alkyl-functional silanes and siloxanes are able to produce hydrophobizing properties on porous mineral substrates such as concrete, brick and lime sandstone, particularly of a few examples. For instance, in particular, monomeric short-chain alkylalkoxysilanes, which are used in solvent-free form (EP 0 101 816 B1), and low-viscosity alkylalkoxysiloxanes having a degree of oligomerization of 3-6 (DE100 56 344 A1) are distinguished by effective impregnation to depth.
It is known that monomeric alkylalkoxysilanes in the course of the application release a certain amount of volatile organic compounds. The VOC content (VOC stands for “volatile organic compounds”) can be reduced if oligomeric silane systems are used, such as alkylalkoxysiloxanes, for example. These have advantages in terms of their use as hydrophobizing agents, since in the course of application they release volatile organic compounds to less of an extent than do monomeric silane systems. These silane systems have the advantage that they achieve a somewhat lower depth of penetration. Moreover, high-viscosity hydrophobizing agents may lead to the discolouration or darkening of the substrate surface or to shiny or oily surfaces, particularly in the case of very dense substrates such as facing concrete, for example.
It is known, furthermore, that architectural preservatives are employed in the form of a solution or in a liquid or pasty or creamy form, in other words as a high-viscosity emulsion (e.g. EP 0 814 110, EP 1 205 481, EP 1 205 505, WO 06/081892; DE1020005004871). When aqueous emulsions and high-viscosity pastes are used that are based on alkylalkoxysilanes, alkylalkoxysiloxanes or blends thereof, the breaking of the emulsion or paste and hydrolysis are followed likewise by formation of alcohols, which are released into the environment as VOCs.
Silicone oils and solutions thereof, which were used as one of the first hydrophobizing agents for protection from penetration by water and pollutants dissolved in water on mineral substrates, do indeed have a low VOC content, but do not exhibit reactivity with the substrate, and obtain only very low depths of penetration. They lead, moreover, to instances of darkening and to sticky surfaces.
In recent years, the emissions guidelines have been tightened in many parts of the world. For example, in one part of the Federal State of California in the USA, a VOC content of less than 100 g/L is required for architectural preservatives. It has to be assumed that these or similar guidelines will become established in other parts of the USA as well and in the rest of the world.
EP 0 579 453B1 describes a process for producing a mixture of monomeric alkoxysilanes, alkoxy-functional siloxanes and silanols and the use thereof for the hydrophobizing of building materials. The partly prehydrolysed mixtures described release alcohols to significantly less of an extent on contact with moisture and on complete hydrolysis.
EP 0 814 110 B1 teaches a process for producing oligomeric mixtures of condensed alkylalkoxysiloxanes which release less VOC. These relatively low-VOC alkylalkoxysilanol oligomers as well were developed in particular for the hydrophobizing of mineral surfaces. The products obtained from them, produced with addition of 1.2 mol H2O/mol Si, lead to darkening of the substrate surface when applied to dense substrates such as facing concrete, for example, and exhibit low depths of penetration.
EP 1 205 481 B1 discloses an oligomers mixture made up of n-propylethoxysiloxanes with different degrees of oligomerization, prepared with addition of 0.65-0.85 mol H2O/mol Si, and the use thereof for the treatment of mineral substrates. The depth of penetration was tested only on lime sandstone, which is significantly more porous than concrete.
U.S. Pat. No. 5,112,393 describes a method for hydrophobizing masonry with low-VOC organoalkoxysilanes, the VOC content of the product according to the invention being 220 g/L.
WO 2013/043725 discloses an emulsion of cyclic methyl siloxanes having a VOC content <100 g/L. However, no detail is given for the substrate tested, and so it is not apparent whether the tests were carried out on concrete plaques (0051) or on mortar cubes (0052).
EP 0 538 555 A1 discloses n-octyltrialkoxysilane-containing aqueous emulsions for the impregnation of mineral building materials. The VOC content is more than 300 g/L. EP 0 340 816A2 and EP 0 412 515A1 also disclose emulsions containing n-octyltriethoxysilane. Owing to the three ethoxy groups per silane module, the VOC content is 350 g/L.
DE 28 49 108 discloses octylmethylcyclotetrasiloxane in connection with the production of silyl phosphates. A process for producing the octylmethylcyclotetrasiloxanes is not disclosed.
In the case of the present invention, the object was to provide a composition which releases volatile organic substances to a very small extent during setting. Furthermore, the compositions of the invention are to be outstandingly suitable for hydrophobizing mineral substrates. Advantageously in that case, small amounts, or significantly smaller amounts, of alcohols are to form as a result of the setting, and so escape into the environment. Furthermore, the compositions are to bring about few or no discolouration on the substrates treated.
Setting in the present context means the possible reaction of alkoxysilyl or hydroxysilyl groups with other reactants, such as with reactive groups of the materials or substrates treated, for example. For instance, pure silicones, without reactive alkoxy groups or hydroxyl groups, are unable to set with a substrate. Therefore, pure silicones are also washed off more readily from the treated substrates, depending on weathering.